Second degree AV block overview

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Cafer Zorkun, M.D., Ph.D. [2]

Overview

Second degree AV block is a disease of the electrical conduction system of the heart. It refers to aconduction block between the atria and ventricles. The presence of second degree AV block is diagnosed when one or more (but not all) of the atrial impulses fail to conduct to the ventricles due to impaired conduction.

Historical perspective

Classification

Second-degree AV block can be of one of the two types: Mobitz I and Mobitz II. Mobitz I, or Wenckebach block , consists of progressive prolongation of PR interval, until loss of conduction to the ventricle occurs (missed beat). Mobitz I block is rarely symptomatic and does not require treatment. On the other hand, Mobitz II AV block is characterized by a constant PR interval with intermittent missed beats. The missed beats can occur with varying frequency such as occasional to 3:1 or 2:1.

Pathophysiology

In Mobitz type I (Wenckebach) there is a progressive prolongation of the PR interval (AV conduction) until eventually an atrial impulse is completely blocked. When an atrial impulse is completely blocked there will be a P wave without a QRS complex. This pattern is often referred to as a “dropped beat.” Mobitz type I occurs because each depolarization results in the prolongation of the refractory period of the atrioventricular (AV) node. When an atrial impulse comes through the AV node during the relative refractory period, the impulse will be conducted more slowly, resulting in a prolongation of the PR interval. Eventually, an impulse comes when the AV node is in its absolute refractory period and will not be conducted. This will manifest on the ECG as a P wave that is not followed by a QRS complex. This non-conducted impulse allows time for the AV node to reset, and the cycle continues. This phenomenon leads to a grouped beating. In Mobitz type II there is a constant PR interval across the rhythm strip both before and after the non-conducted atrial beat. Each P wave is associated with a QRS complex until there is one atrial conduction or P wave that is not followed by a QRS. Mobitz type II is often a problem in the infra-nodal conduction system, and therefore, is associated with a widened QRS complex, bundle-branch block, or fascicular block. Be aware that if more than one P wave is not conducted this is no longer a Mobitz type II and is considered a high degree AV block.

Causes

Differentiating second degree AV block from Other Diseases

Epidemiology and Demographics

Risk Factors

Common risk factors associated with second degree AV block include Intrinsic atrioventricular node disease, Myocarditis , Acute myocardial infarction, Prior cardiac surgery, Older age,Heart attack or coronary artery disease, Cardiomyopathy, Sarcoidosis, Lyme disease, High potassium levels, Severe hypothyroidism, Certain inherited neuromuscular diseases, Medicines that slow the heart rate , After open heart surgery.

Screening

Natural History, Complications, and Prognosis

Patients with Mobitz type II second degree AV block who are hemodynamically stable do not require urgent therapy with atropine or temporary cardiac pacing. However, Mobitz type II second degree AV block is by nature unstable and frequently progresses to third degree (complete) AV block, so patients should be continuously monitored with transcutaneous pacing pads in place in the event of clinical deterioration. While stable patients are being monitored, reversible causes of Mobitz type II second degree AV block such as myocardial ischemia, increased vagal tone, hypothyroidism, hyperkalemia, and drugs that depress conduction, should be excluded in patients prior to implantation of a permanent pacemaker. For patients with Mobitz type II second degree AV block who do not have a reversible etiology, we recommend implantation of a permanent pacemaker (Grade 1A). We implant a dual chamber DDD pacemaker whenever possible in an effort to maintain physiologic AV synchrony.

Diagnosis

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